My present invention relates to a spreading apparatus for the spreading of a layer of a spreadable material, especially wood chips, wood fibers, sawdust or the like upon a spreading conveyor belt upon which the layer is to be formed. More particularly, the invention relates to a spreading apparatus for use in the production of mats or the like of the spreadable material, with or without a binder, for use in the production of particleboard, fiberboard or generally wood material boards and where the spreading apparatus can have a storage bin associated with a dosing or metering device by means of which the spreadable material is deposited on the conveyor belt.
In such apparatus it is common to provide the dosing device so that it is located above the conveyor belt and at the bottom of the storage bin and can include a spreading roller array which has at least one fine-material spreading portion and, downstream thereof in the direction of travel of the deposited layer and the material to be spread on the array, a coarse-material spreading portion.
A prior art spreading apparatus of this type has the spreading layer array so constructed and arranged that it separates a spreadable material. At the beginning of the spreading roller array and thus in the fine-material spreading portion, fine material from the bin, for example to form a cover layer in the mat or pressed board, is initially deposited while at the end the spreading roller array or stretch and thus in the coarse-material spreading region, practically only coarse material from the bin is deposited upon the conveyor to form a middle layer.
The spreadable material usually consists of glue-covered particles. However, the bonding of the coarse material in the middle layer is relatively small in spite of the presence of the adhesive. As a consequence, the transverse tensile strength of the particleboard (chipboard or fiberboard or like wood material boards) which are made from the mats can be limited.
To avoid the reduction in the transverse tensile strength can result from the limited bonding in the coarse material layer, the spreading apparatus can be made with a spreading roller stretch having upper and lower parts and the latter can be located in the lower third of the spreading stretch beneath the upper part. Thus, the upper part can initially deposit fine material on the conveyor and this fine layer can be measured by a coarse-material middle layer or a mixture of fine material and coarse material to form the middle layer. In a subsequent part of the spreading stretch, initially a mixture of coarse material and fine material can be deposited to complete the middle layer. Only fine material can be deposited to provide an upper cover layer.
This system, while it can partially solve the problem mentioned previously, has the drawback that the fine material distribution in the middle layer is unsatisfactory and furthermore, because of the separation of the layer deposition between upper and lower parts, the spreading apparatus itself is of expensive construction and can be expensive to operate.
It is, therefore, the principal object of the present invention to provide a spreading apparatus of the type which comprises a conveyor belt adapted to receive the layer of spreadable material, a storage bin for the spreadable material and dosing device subdivided along its roller array into a fine-material spreading part and a coarse-material spreading part, whereby the disadvantages of earlier systems are avoided.
A more specific object of this invention is to provide a spreading device in which the middle layer can have an especially fine distribution of the fine material thereof so that the transverse tensile strength of the resulting board can be increased without increasing the production cost or the capital cost for the spreading apparatus.
Yet another object of the invention is to provide an improved spreader which can have advantages for the production of mats or layers of particles for use in the production of particleboards with continuous or cycling presses.
These objects are achieved in accordance with the invention with a spreader for producing a layer of spreadable material and especially wood chips, wood fibers, sawdust or other wood particles for the production of wood particleboard, in which the spreader comprises:
a conveyor belt adapted to receive a layer of a spreadable material;
a storage bin above the belt and containing a quantity of spreadable material;
a dosing device receiving spreadable material from the storage bin and depositing spreadable material in a layer on the conveyor belt, the dosing device comprising an array of mutually parallel spreading rollers extending generally transversely of a direction of displacement of the belt and between which the spreadable material is scattered onto the belt, the spreadable material passing over the array of rollers, the array of rollers including a fine-material spreading portion within which a fine material fraction of the spreadable material is scattered onto the belt and a coarse-material spreading portion in which a coarse material fraction of the spreadable material is scattered onto the belt; and
a drive for driving at least one of the spreading rollers in the coarse-material spreading portion at an angular velocity higher by a predetermined amount than that the spreading rollers of the fine-material spreading portion.
According to the invention, therefore, the coarse-material spreading portion has at least one roller of the array and, advantageously, a plurality of rollers of the array which are driven at an angular velocity which is higher than the angular velocity of the rollers of the fine-material spreading portion by a predetermined degree. As a consequence of the coarse-material rollers being driven at a higher speed, a portion of the coarse material is scattered opposite the direction of travel of the spreadable material and back into the fine-material spreading part.
This results in an especially intimate mixture between fine material and coarse material below the fine material spreading part and the production of a middle layer or at least a part or half of the middle layer which will overlap and intimately blend with the fine material of the fine-material part.
The result is that the fine material has the function of a binder for the coarse material and such that the intimate mixture of the fine material and coarse material which takes place where the coarse material is drawn back onto the fine material has an especially fine distribution of any binder which may be provided on the particle. The result is increased transverse tensile strength of the pressed particleboard produced in the process.
The spreader of the invention thus permits the formation of a cover layer or the like substantially exclusively from the fine material and an intermediate layer which at least initially is a result of an intimate mixing of fine material and coarse material or can be a fine material/coarse material mixture throughout. However, the spreader of the invention can be provided as a middle layer spreader for use when the lower cover layer and, if desired, an upper cover layer, are provided by separate fine material spreaders. In this case, the middle layer is formed at least in part as an intimate mixture of fine and coarse material.
The apparatus for producing the mat can include as many spreaders as may be necessary, and for example, the spreader of the invention can be used upstream of or downstream of further intermediate layer spreaders or for the generation of an intermediate layer half and other spreaders can be provided for applying the upper and lower cover layers or, if desired, even additional intermediate layers of the fine material.
The coarse material portion of the array of rollers may have a plurality of rollers operated at the increased speed and these can alternate with rollers which are operated at the speed of the fine-spreader rollers.
The means for casting the coarse particles back in the direction opposite that in which the particles are traveling on the array of rollers can include baffles which are located below the higher speed coarse-material rollers.
According to a feature of the invention, these baffles have surfaces curved to correspond to the curvature of the higher-speed coarse material rollers and spaced at a fixed distance from them. The spacing between the baffle surfaces and the high-speed spreading rollers and the curvatures can be so selected that an optimum scattering of the coarse particles and an optimum degree of spread thereof backwards can be achieved. What is important of course is that the backward scattering of coarse particles reaches at least to a region below the fine-material spreading parts.
The provision of a plurality of higher speed coarse-spreading rollers, preferably alternating with slower speed rollers in the coarse-spreading part of the roller array, can ensure that various mats of spreadable material, for example, chips, fibers and sawdust, can be used with assurance that the desired distribution of coarse and fine particles will be achieved.
The provision of slower-speed rollers between the higher-speed rollers has the advantage that it permits further fine-material scattering on the one hand and on the other hand a reduction or avoidance of intersection of the backscattering trajectories of the particles.
It has been found to be advantageous, moreover, to increase the spacing between the fine-spreader rollers toward the end of the fine-spreader part of the array or over the entire length thereof so that at least at the upstream end of this part of the array, there is practically an exclusive spreading of the fine material while toward the end of that part of the array, both fine material and a fine-particle fraction of the coarse material will begin to deposit together.
It has been found to be advantageous, moreover, to drive the higher-speed rollers with a speed which is 2 to 12 times higher than the speed of the rollers of the fine-particle part and thereby ensure the desired degree of scattering of the coarse particles. Materials which are undesirable in the mat, for example, particles of too large a size, metal particles, clumps of glue or mixtures of glue with particles may be collected at the end of the roller array.
It is also possible to operate with speeds of higher-speed rollers which are 5 to 40 times greater than the speeds of the fine-particle spreader rollers and the fine-particle spreader rollers can have speeds of 5 RPM to 25 RPM by way of example. The higher-speed rollers can be driven at speeds of 150 RPM to 200 RPM. The speeds of the rollers of the parts of the array may be adjustable and coupled with one another, and with the speed of the conveyor belt and/or with the speed of a feed roller depositing the spreadable material on the array of rollers and/or a feed belt at the bottom of the bin. Thus when the conveyor spreader rollers can operate at respective speeds in which at least the coarse-spreader rollers can be automatically driven at a speed higher by the aforementioned factor than the fine-spreader rollers.